(i) Field of the Invention
This invention relates generally to modified polyethylene terephthalate polymers and copolymers that may be processed into plastic containers using conventional extrusion blow molding equipment. More particularly, it relates to polymers of the branched and end-capped type, which polymers contain substantially fewer gels. It also relates to the use of specific end-capping agents for making such polymers in a reproducible manner.
(ii) Prior Art
U.S. Pat. No. 4,161,579 (Edelman 1), hereby incorporated by reference, discloses a large number of modified polyethylene terephthalate polymers (hereinafter modified PET polymers) that can be successfully processed to form a hollow container by conventional extrusion blow molding techniques using existing, conventional, extrusion blow molding equipment. The patent also describes the requirements for polymers suitable for extrusion blow molding. Essentially, such polymers must have (1) "high zero shear rate melt viscosity", and "absence of gels", in order to make a satisfactory "parison" after extrusion of the polymer from the annular die; and (2) sufficiently high "shear sensitivity", in order to be capable of being forced through the melt extrusion equipment without generating excessive pressure. All the polymers disclosed in Edelman 1 involve the use of branching agents and end-capping agents. However, as stated therein, the chain terminating agent "must have a boiling point above 200.degree. C." (see Edelman 1, column 10, lines 30-34) and also is a monofunctional compound containing one --COOH group (or its ester).
U.S. Pat. No. 4,234,708 (Edelman 2) is similar to Edelman 1, except that it relates to a modified polyethylene iso/terephthalate copolymer. Edelman 2 also points out that it is necessary to avoid the presence of excessive amounts of isophthalic acid or dimethyl isophthalate as starting materials for the copolymers, as this will result in the formation of prepolymers which are totally amorphous. Further, such prepolymers possess a particularly low glass transition temperature. They therefore tend to stick together if solid state polymerization is attempted at normally employed temperatures (in order to increase molecular weight and give sufficiently "high zero shear rate melt viscosity"). This, in turn, reduced the surface area from which glycol can evaporate. Also, Example H shows that when a copolymer is prepared from terephthalic acid and isophthalic acid in a ratio of 75:25 by weight, the prepolymer is completely amorphous and therefore is not capable of solid-state polymerization to increase molecular weight. (See Edelman 2 at column 10, lines 15-31 and column 23, lines 27-34.)
U.S. Pat. No. 4,219,527 (Edelman 3) is similar to Edelman 1, except that it is directed to a blow molding process.
Defensive Publication No. T954,005, published Jan. 4, 1977, discloses a process for preparing containers, such as bottles, using a branched polyester of terephthalic acid, a combination of ethylene glycol and 1,4-cyclohexanedimethanol and a small amount of a polyfunctional branching compound. A molten parison of such polyester is extruded and then expanded in a container mold to form the desired container. More specifically, the polyester used for that invention can be broadly described as comprised of terephthalic acid and a diol component comprised of 10 to 40 mole percent 1,4-cyclohexanedimethanol and 90 to 60 mole percent ethylene glycol and a polyfunctional branching compound (see unexamined application at page 4, lines 18-21). Examples of the polyfunctional branching compound are given at page 4, lines 22-31 of the unexamined application, and include pentaerythritol, and trimethylolpropane. In addition, the unexamined application states the following at page 5, lines 14-31 concerning the possible use of chain terminators to prevent the formation of gels in the polyesters. "The rapid buildup of molecular weight can produce a nonuniform gel in the polyester. To control this rapid rate of polymerization and to obtain the desired degree of polymerization, it is often desirable to use a chain terminator in accordance with techniques well known in the art. By using the proper level of chain terminator the polymer can be stopped at the desired degree of polymerization. Useful terminators include monofunctional acids, esters or alcohols. It is often desirable to use a relatively nonvolatile terminator since the terminator can be lost by volatilization during polycondensation. Examples of terminators that can be used include heptadecanoic acid, stearic acid, nonadecanoic acid, benzoic acid, phenylacetic acid, 4-biphenylcarboxylic acid, phenyloctadecanoic acid, 1-heptadecanol, 1-octadecanol, and 1-nonadecanol. Lower molecular weight terminators such as acetic acid, propionic acid, methanol and ethanol can also be used."
Some of the chain terminators suggested in the defensive publication, e.g. methanol and ethanol, are well known to have boiling points far below 200.degree. C. However, nowhere does the defensive publication disclose a solid-phase polymerization process. Further, it is believed that at least most of the copolymers described in the defensive publication are so-called "amorphous" polymers, and as such would be incapable of being commercially solid-phase polymerized to give polymers of very high molecular weight.
Essentially, for the purpose of the invention claimed hereinafter, the prior art does not appear to disclose an end-capping agent for PET polymer or copolymer that has a boiling point or sublimation point less than above about 200.degree. C. and is also an aromatic monocarboxylic acid wherein any of the remaining end groups are alkoxy groups containing from one to ten carbon atoms. Even less does such art relate to the relative effectiveness of isomers of such compounds.